Pure and supported chromia catalysts are used for a variety of catalytic transformations. High surface area is an important characteristic of these materials. High surface area chromia-based materials are used as catalysts for halogenations of hydrocarbons (especially fluorinations), the dehydration of alcohols, the dehydrogenation of alkanes and olefins, and isomerization reactions. Presently, high surface area chromia is made through the reduction of CrO3 with methanol under high temperature supercritical methanol conditions (˜300° C.). This method is dangerous because of the extreme conditions for synthesis and the mixing of a powerful reducing agent, e.g., CrO3, with methanol. It is also very expensive due to the equipment needed to execute the supercritical extraction. Thus, a need exists for a method for producing high surface area chromia catalysts, which does not use a dangerous mixture of chemicals nor utilize a very high temperature supercritical extraction process.
Aerogel and xerogel materials containing high surface area chromia can be produced using sol-gel chemistry methods. Sol-gel chemistry is an attractive alternative to other synthetic methods for many reasons. The method is low temperature, low cost, and can generally be done under ambient conditions with general lab equipment, all of which make processing convenient and inexpensive. Historically, the sol-gel method has employed the use of metal alkoxide precursors that readily undergo catalyzed hydrolysis and condensation to form a sol of metal oxide particles with nanoscale dimensions (1–100 nm).
Skapin et al, J. Non-Cryst. Solids 1998, 225, 163 and Armor et al. J. Appl. Catal. 1985, 19, 327 report on a synthetic method to prepare chromia aerogel powders. An aqueous suspension of CrO3 is added to methanol, followed by supercritical processing to chemically reduce the Cr(IV) to Cr(III) and form chromia. Surface areas of the chromia aerogels reported by Skapin et al were 300–550 m2/g and Armor et al were 500–700 m2/g. Chromium trioxide is a powerful reducing agent and great care must be taken to avoid a violent reaction with alcohol. The high temperature and pressures of this process require the use of expensive and sophisticated processing equipment. This fact coupled with the dangerous nature of the precursor solutions, that require extreme safety precautions, likely precludes the widespread application of this method.
With the particular exceptions of titanium and zirconium, alkoxide-transition metal precursors are not readily available. In the absence of the alkoxides, researchers have used more traditional approaches for the successful sol-gel preparation of transition metal oxides. These approaches typically involve aqueous precipitation of the metal ion with base, extensive washing and solvent exchange of the products and atmospheric or supercritical drying. For example, vibrant chromia (the generic term for hydrated chromic oxide of unknown hydration) gels have been prepared by such methods. These methods have been clearly summarized by Livage et al., Prog. Inorg. Chem., 18, 259 (1988).